In order to improve the exhaust gas characteristics and fuel cost performance of a motor vehicle, it is known to control the air-fuel ratio of an air-fuel mixture to be supplied to an internal combustion engine, to have a value on the fuel-lean side of a theoretical air-fuel ratio, thereby allowing the engine to be operated in a lean-combustion mode. If the engine is continually operated in the lean-combustion mode, however, the engine output will be insufficient in some specific operating states (e.g., sudden-departure operating state and sudden-acceleration operating state). Conventionally, therefore, the air-fuel ratio is adjusted to the theoretical air-fuel ratio or its near value (hereinafter referred to as near theoretical air-fuel ratio) to effect rich-combustion operation in the specific operating states of the engine.
According to the conventional air-fuel ratio control, however, the amount of fuel supply to the engine sometimes may undergo a sudden change when the air-fuel ratio is changed between the near theoretical air-fuel ratio and an air-fuel ratio on the fuel-lean side thereof, in order to switch the combustion mode between rich combustion and lean combustion. In such a case, the engine output, which depends on the fuel supply amount, thermal efficiency of the engine, etc., changes suddenly. For example, when the accelerator pedal is released from a driver's stepping operation so that an acceleration operating state (one of the specific operating states) is finished, conventional air-fuel ratio control apparatuses operate to reduce the fuel supply amount rapidly, in order to restore the air-fuel ratio to a value on the fuel-lean side. As a result, the engine output lowers suddenly. If the engine output undergoes such a sudden change, a torque shock is caused in the engine, thus retarding the drivability.
Disclosed in Published Unexamined Japanese Patent Application No. 2-267340 is an air-fuel ratio control apparatus which, belonging to the aforementioned conventional apparatuses in a broader sense, is so designed that the fuel supply amount is reduced during the transition to the lean operating state.